WtfJS

function manipulateArray(array, commands) {
    const add = (index, element) => {
        array.splice(index, 0, element);
    }

    const addMany = (inputOne, inputTwo) => {
        inputOne = Number(inputOne);

        inputTwo = inputTwo.map(Number);

        array.splice(inputOne, 0, ...inputTwo);
    }

    const contains = number => {
        if (array.includes(number)) {
            let index = array.indexOf(number);
            console.log(index);
        } else {
            console.log(-1);
        }
    }

    const remove = index => {
        array.splice(index, 1);
    }

    const shift = position => {
        for(let i = 0; i < position; i++) {
            array.push(array.shift());
        }
    }

    const sumPairs = () => {
        array = array.map((e, i, array) => i < array.length - 1 ? (array[i] += array[i + 1]) : array[i] = array[i]).filter((e, i) => i % 2 === 0);
    }

    for (let currentCommand of commands) {
        let wordsInCommand = currentCommand.split(' ');
        let firstWord = wordsInCommand.shift();

        if (firstWord == 'add') {
            let index = Number(wordsInCommand[0]);
            let element = Number(wordsInCommand[1]);
            add(index, element);
        } else if (firstWord == 'addMany') {
            let index = Number(wordsInCommand.shift());
            addMany(index, wordsInCommand);
        } else if (firstWord == 'contains') {
            let element = Number(wordsInCommand[0]);
            contains(element);
        } else if (firstWord == 'remove') {
            let index = Number(wordsInCommand[0]);
            remove(index);
        } else if (firstWord == 'shift') {
            let position = Number(wordsInCommand[0]);
            shift(position);
        } else if (firstWord == 'sumPairs') {
            sumPairs();
        } else if (firstWord == 'print'){
            array = array.join(', ');
            console.log(`[ ${array} ]`);
            break;
        }
    }
}